Just some red flags I now see because of experience:

  1. Having an opinion on every fucking topic.

If you are dating someone and you realise that person always has his opinion on every matter, run. It's just not possible because you have to have knowledge and information to base your opinion on. If he has something to say to everything it can be a sign of narcissism or at least a big ego. He thinks he is extremely important and can't shut up. It's not possible for him to say "sorry, I have no opinion about that I first have to read more about xy". Otherwise his massive fragile ego would be hurt.

  1. Having no opinions at all / not telling his opinions / always agreeing with you.

First of all, it's boring. He has a boring character and you might lose interest even of he is nice. There won't be any mature discussion or talking points, which are important. Second: It also can be a red flag because if someone doesn't tell you his opinion or doesn't have any he could easily hide something from you. Be careful.

There always has to be a balance!